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Friday, October 21, 2016

Neural basis of auditory entrainment

In PsyPost:

The auditory brainstem responds more consistently to regular sound sequences than irregular sound sequences, according to a recent study published this September in Neuroscience. The study adds to our understanding of how the brain processes regular sound patterns. [...]

The study, by Alexandre Lehmann (University of Montreal and McGill University), Diana Jimena Arias (University of Quebec at Montreal) and Marc Schönwiesner of (University of Montreal), traced entrainment-related processes along the auditory pathway. They simultaneously recorded the EEG (electrical activity in the brain) responses of 15 normal-hearing participants in their brainstem and cortex whilst regular and irregular sequences of a sound were played. There were then random omissions during the sequences.

The results revealed that the auditory cortex responded strongly to omissions whereas the auditory brainstem did not. However, auditory brainstem responses in the regular sound sequence were more consistent across trials than in the irregular sequence. They also found stronger adaptation in the cortex and brainstem of responses to stimuli that preceded omissions than those that followed omissions.

This makes sense. I'm thinking in particular of the observations that William Condon made about neonatal entrainment. That implied brain stem mediation as the auditory cortex is not mature at birth.

Abstract: Neurons in the auditory cortex synchronize their responses to temporal regularities in sound input. This coupling or "entrainment" is thought to facilitate beat extraction and rhythm perception in temporally structured sounds, such as music. As a consequence of such entrainment, the auditory cortex responds to an omitted (silent) sound in a regular sequence. Although previous studies suggest that the auditory brainstem frequency-following response (FFR) exhibits some of the beat-related effects found in the cortex, it is unknown whether omissions of sounds evoke a brainstem response. We simultaneously recorded cortical and brainstem responses to isochronous and irregular sequences of consonant-vowel syllable /da/ that contained sporadic omissions. The auditory cortex responded strongly to omissions, but we found no evidence of evoked responses to omitted stimuli from the auditory brainstem. However, auditory brainstem responses in the isochronous sound sequence were more consistent across trials than in the irregular sequence. These results indicate that the auditory brainstem faithfully encodes short-term acoustic properties of a stimulus and is sensitive to sequence regularity, but does not entrain to isochronous sequences sufficiently to generate overt omission responses, even for sequences that evoke such responses in the cortex. These findings add to our understanding of the processing of sound regularities, which is an important aspect of human cognitive abilities like rhythm, music and speech perception